Elastic nonwoven fabrics made from thermoplastic polyurethane elastomers (hereinafter “TPU”) proposed so far have been used in applications including garments, hygiene materials and materials for sporting goods due to their high elasticity, low residual strain and superior breathability.
Meltblowing is a typical process for producing elastic nonwoven fabrics from TPU. Meltblown elastic nonwoven fabrics exhibit high elasticity, flexibility and breathability, and therefore they have been used in relatively active applications that require conformity to body movements, such as side bands in disposable diapers, gauze pads in adhesive bandages, and disposable gloves.
JP-A-7-503502 discloses a spunbonded nonwoven fabric comprising a web of elastomeric thermoplastic substantially continuous filaments. This spunbonded nonwoven fabric is mentioned to have a more pleasant feel than meltblown nonwoven fabrics because they more closely approximate textile fiber diameters and consequently textile-like drape and hand. JP-A-7-503502 describes thermoplastic polyurethane elastomers as the thermoplastic elastomers, but it is not disclosed starting temperatures for solidifying of these elastomers and particle number of polar-solvent insolubles. As will be illustrated in Comparative Examples 1 and 2 of this specification, fibers will break and adhere to one another during spinning when the thermoplastic elastomer has a starting temperature for solidifying of less than 60° C., or contains over 3.00×106 particles of polar-solvent insolubles per g of the elastomer; the result is a nonwoven fabric having bad touch.
JP-A-9-87358 discloses a thermoplastic polyurethane resin that contains, per g of the resin, 2×104 or less particles of polar-solvent insolubles ranging from 6 to 80 μm in particle diameters. This thermoplastic polyurethane resin has been shown to be useful for producing elastic polyurethane fibers without causing any increase in nozzle back pressure and any filament breakage during the melt spinning. The present inventors have tried to produced the thermoplastic polyurethane resin according to JP-A-9-87358, but they cannot obtain it.
JP-A-2002-522653 addresses the characteristic “sticky” nature of the thermoplastic elastomers as one of the problems encountered in spunbonding the elastomers into nonwoven fabrics. It has been pointed out that turbulence in the air can bring filaments into contact and they can adhere to one another in the spunbonding. The “stickiness” has been proven to be especially troublesome during rolling up of the webs. Further, JP-A-2002-522653 mentions breakage and elastic failure of the strand during extrusion and/or stretching. As will be illustrated in Comparative Example 2 of this specification, spinning TPU (Elastollan 1180A (BASF Japan Ltd.)) described in JP-A-2002-522653 is accompanied with filament breakage and the resultant nonwoven fabric is unsatisfactory.
WO99/39037 discloses an elastic nonwoven fabric comprised of a thermoplastic polyurethane resin that has a hardness (JIS-A hardness) of 65 A to 98 A and a fluidization initiation temperature of 80 to 150° C. This nonwoven fabric is obtained by stacking continuous filaments of a thermoplastic polyurethane resin into a sheet form and fusion-bonding the stacked filaments at the contact points by their own heat. This production is the meltblowing. The present inventors performed the procedure described in WO99/39037 to prepare a thermoplastic polyurethane resin and used it in Comparative Example 4 to form a spunbonded nonwoven fabric. The result was filament breakage during the spinning and the resultant nonwoven fabric was of inferior quality.
JP-A-9-291454 discloses elastic nonwoven fabrics, having excellent drape, comprising a conjugate fiber comprising a crystalline polypropylene and a thermoplastic elastomer. It discloses an elastic nonwoven fabric which comprises a concentric sheath-core conjugate fiber made up of 50 wt % of a urethane elastomer as the core and 50 wt % of a polypropylene as the sheath (Example 6). The disclosure extends to an elastic nonwoven fabric which comprises a conjugate fiber made up of 50 wt % of a urethane elastomer and 50 wt % of a polypropylene to show a six-segmented cross section (Example 8). These nonwoven fabrics are produced by opening staple fibers with a carder and heating them with a through-air dryer. They are capable of about 75% elastic recovery after 20% elongation and have excellent drape. However, they are still insufficient in elastic properties for applications such as garments, hygiene materials and materials for sporting goods.